1. Field of the Invention
This invention relates to a production process of chlorine, and more specifically to a process for producing chlorine by oxidizing an offgas, which has been discharged from a reaction step of an organic compound and containing hydrogen chloride, with an oxygen gas.
2. Description of the Prior Art
Hydrogen chloride is byproduced in large amounts in both chlorination and phosgenation of organic compounds. It is however thrown away without utilization because the production of byproduced hydrogen chloride is far greater than the market demand for hydrogen chloride. Large expenses are also required to treat byproduced hydrogen chloride for its disposal.
The reaction in which hydrogen chloride is oxidized to produce chlorine has been known as "Deacon reaction" for many years. The copper-base catalyst which was invented for the first time by Deacon in 1868 has conventionally been considered to exhibit the best activities. Since then, a number of so-called Deacon catalysts with various compounds added as a third component to copper chloride and potassium chloride have also been proposed. Catalytic reactions making use of these catalysts however required high temperatures of 400.degree. C. and higher. In addition, the above Deacon catalysts were insufficient in service life.
Regarding the process for producing chlorine by using a Deacon catalyst, a proposal has been made in U.S. Pat. No. 4,394,367. According to the process of this U.S. patent, a gas obtained as a result of the catalytic reaction is introduced in a sulfuric acid absorption column the temperature of which is slightly lower than the reaction temperature, so that the gas is dewatered and dried. Subsequent to its compression, impurities such as polychlorinated byproducts are extracted and removed with carbon tetrachloride and chlorine is then liquefied for its separation.
The above-proposed process however requires the recirculation and utilization of a great deal of sulfuric acid for the elimination of resultant water contained in a large amount in the thus-formed gas, because the sulfuric acid absorption column is operated at a high temperature near about 200.degree. C. The above process cannot therefore be considered as an advantageous process in every aspect, when the initial cost and power cost are both taken into consideration.
As an improvement to the Deacon process, it is also disclosed in "The Chemical Engineering", 229 (1963) that after conducting the reaction by using air as an oxidizing agent, the resultant gas is washed with water to recover hydrogen chloride as 30% hydrochloric acid, the remaining portion of the resultant gas is dewatered and dried with sulfuric acid, and carbon tetrachloride is used as an extracting reagent for chlorine.
According to the above-proposed process, air is used as an oxygen source. The concentration of chlorine in the resultant gas is therefore low, leading to the need for a considerable power cost for the liquefaction and separation of chlorine.
As a drawback common to both of these processes, the use of a solvent such as carbon tetrachloride in the separation step after the reaction is mentioned. As a result, a cumbersome step is additionally required for the separation of chlorine and carbon tetrachloride from each other. When the remaining gas is recycled after separation of chlorine as in the process of this invention, the solvent is mixed in the recycled gas so that the chromium oxide catalyst is adversely affected.
Various proposals have also been made as to the use of chromium oxide, which is other than the aforementioned Deacon catalysts, as a catalyst. None of such proposals however indicate sufficient activities. For example, U.K. Patent No. 676,667 discloses to support CrO.sub.3 on an alumina carrier, followed by calcining or reduction with hydrogen so that a trivalent chromia catalyst is formed. It however contains low conversions only. Although these chromium oxide catalysts can achieve high initial conversions, their catalytic activities are reduced significantly along the passage of time. As an improvement to this problem, it is proposed in U.K. Patent No. 846,832 to incorporate chromyl chloride in the raw material, i.e., hydrogen chloride in order to maintain the high conversions.